1. Field of the Invention
This invention relates to a metal chloride gas generator, a hydride vapor phase epitaxy growth apparatus, and a method for fabricating a nitride semiconductor template.
2. Description of the Related Art
Gallium nitride compound semiconductors, such as gallium nitride (GaN), aluminum gallium nitride (AlGaN), and indium gallium nitride (InGaN) have attracted attention as light-emitting device materials capable of red through ultraviolet light emission. One growing method for these gallium nitride compound semiconductor crystals is a Hydride Vapor Phase Epitaxy (HVPE) growing method using metal chloride gas and ammonia as raw material.
A feature of the HVPE method is as follows. According to this method, it is possible to obtain a growth rate of 10 μm/hr to 100 μm/hr or higher which is remarkably higher than a typical growth rate of several μm/hr in other growing methods such as Metal Organic Vapor Phase Epitaxy (MOVPE) and Molecular Beam Epitaxy (MBE). For this reason, the HVPE method has been often used in the manufacture of a GaN free-standing substrate (see e.g. JP Patent No. 3886341) and an AlN free-standing substrate. Here, the term “free-standing substrate” refers to a substrate having such strength to hold its own shape and not to cause inconvenience in handling.
In addition, a light-emitting diode (LED) made of a nitride semiconductor is typically formed over a sapphire substrate. In its crystal growth, after a buffer layer is formed over a surface of the substrate, a GaN layer having a thickness of the order of 6 to 15 μm including an n-type layer is grown thereover, and an InGaN/GaN multiple quantum well light-emitting layer (several hundreds nm thick in total) and a p-type layer (200 to 500 nm thick) are grown thereover in this order. The GaN layer under the light-emitting layer is thick in order to improve the crystallinity of GaN on the sapphire substrate and the like. This is followed by electrode formation, resulting in a final device structure as shown in FIG. 11 which will be described later. In the case of growth with the MOVPE method, the crystal growth typically requires about 6 hours, and about half of 6 hours is the time required to grow a so-called “template portion” that are GaN layers under the light-emitting layer.
From the above, it is supposed, if it is possible to apply the HVPE method with the remarkably high growth rate to the growth of the template, it will be possible to substantially shorten the growth time, thereby dramatically reduce LED wafer manufacturing cost. In growing the template portion with the HVPE method which can lower the production cost, however, due to contamination by many unintended impurities, it is difficult to fabricate the good quality template.
For the HVPE apparatus used for manufacturing the nitride semiconductor, Ga, NH3 gas, HCl gas are generally used as main raw material. In addition, the growth temperature required for effectively forming a film is a high temperature, namely, not lower than 1000 degrees Celsius. For this reason, a material to be used for a gas inlet pipe and a reactor is e.g. quartz that is chemical resistant and heat resistant to NH3 gas and HCl gas that are highly reactive at high temperature. Specifically, the HVPE apparatus has a structure as shown in FIG. 12 which will be described later, and has a tube reactor made of quartz divided into a raw material section on its upstream side (i.e. upstream raw material section) and a growing section on its downstream side (i.e. downstream growing section), and an upstream open end of the reactor is closed by an upstream flange made of stainless steel (SUS), and the gas inlet pipes made of quartz are installed through the upstream flange from the raw material section towards the growing section. Because the gas inlet pipes made of quartz cannot be attached directly to the upstream flange, a pipe made of SUS is connected to an outer side of an upstream end of each of the gas inlet pipes, and this pipe is attached to the upstream flange (see e.g. JP-A-2002-305155). This technique is very ordinary, and it would not be an exaggeration to say that all HVPE apparatuses utilize this technique.